There are various kinds of metal oxides, which are used for a wide range of applications. Indium oxide is a well-known material and used for a material of a transparent electrode which is needed in a liquid crystal display or the like.
Some metal oxides have semiconductor characteristics. The metal oxides having semiconductor characteristics are one kind of compound semiconductor. The compound semiconductor is a semiconductor formed using two or more kinds of atoms bonded together. In general, metal oxides become insulators; however, it is known that metal oxides become semiconductors depending on the combination of elements included in the metal oxides.
For example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are known to show semiconductor characteristics among metal oxides. A thin film transistor in which a transparent semiconductor layer formed of such a metal oxide is used for a channel formation region is disclosed (Patent Documents 1 to 4 and Non-Patent Document 1).
As the metal oxides having semiconductor characteristics, not only the above-described mono-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m is a natural number) having a homologous series is a known material (Non-Patent Documents 2 to 4).
Further, it is proved that the homologous thin film described above can be used for a channel layer of a thin film transistor (Patent Document 5 and Non-Patent Documents 5 and 6).
In addition, Patent Document 6 and Patent Document 7 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a metal oxide semiconductor and such a transistor is used as a switching element or the like of an image display device.